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/*!
@file
Forward declares `boost::hana::product`.
@copyright Louis Dionne 2013-2017
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
*/
#ifndef BOOST_HANA_FWD_PRODUCT_HPP
#define BOOST_HANA_FWD_PRODUCT_HPP
#include <boost/hana/config.hpp>
#include <boost/hana/core/when.hpp>
#include <boost/hana/fwd/integral_constant.hpp>
BOOST_HANA_NAMESPACE_BEGIN
//! Compute the product of the numbers of a structure.
//! @ingroup group-Foldable
//!
//! More generally, `product` will take any foldable structure containing
//! objects forming a Ring and reduce them using the Ring's binary
//! operation. The initial state for folding is the identity of the
//! Ring's operation. It is sometimes necessary to specify the Ring to
//! use; this is possible by using `product<R>`. If no Ring is specified,
//! the structure will use the Ring formed by the elements it contains
//! (if it knows it), or `integral_constant_tag<int>` otherwise.
//! Hence,
//! @code
//! product<R>(xs) = fold_left(xs, one<R or inferred Ring>(), mult)
//! product<> = product<integral_constant_tag<int>>
//! @endcode
//!
//! For numbers, this will just compute the product of the numbers in the
//! `xs` structure.
//!
//! @note
//! The elements of the structure are not actually required to be in the
//! same Ring, but it must be possible to perform `mult` on any two
//! adjacent elements of the structure, which requires each pair of
//! adjacent element to at least have a common Ring embedding. The
//! meaning of "adjacent" as used here is that two elements of the
//! structure `x` and `y` are adjacent if and only if they are adjacent
//! in the linearization of that structure, as documented by the Iterable
//! concept.
//!
//! @note
//! See the documentation for `sum` to understand why the Ring must
//! sometimes be specified explicitly.
//!
//!
//! Example
//! -------
//! @include example/product.cpp
#ifdef BOOST_HANA_DOXYGEN_INVOKED
constexpr auto product = see documentation;
#else
template <typename T, typename = void>
struct product_impl : product_impl<T, when<true>> { };
template <typename R>
struct product_t;
template <typename R = integral_constant_tag<int>>
constexpr product_t<R> product{};
#endif
BOOST_HANA_NAMESPACE_END
#endif // !BOOST_HANA_FWD_PRODUCT_HPP
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